The self-declared caliphate has also suffered financially from coalition strikes on the oil facilities which provide much of its income and has lost territory in Iraq to the forces of the Baghdad government.

Prime Minister David Cameron 's official spokeswoman said: "We have now seen more than 25,000 Daesh fighters killed, over 600 in the last month or so.

"The number of Daesh fighters is estimated to be at its lowest for about two years."